The present invention relates to a process and apparatus for continuously monitoring concentrations of gaseous components in gas mixtures, with the exception of O.sub.2.
The measurement of oxygen as a component in a gas mixture with the aid of a solid body which contains a solid body electrolyte and platinum electrodes has long been known. In so doing, air is most often utilized as the reference gas. The upper surface of one side of the solid electrolyte is placed in contact with the gas mixture to be measured, and the upper surface of the other side of the solid electrolyte is placed in contact with air. The differential of the oxygen partial pressures then creates an electrical signal whose magnitude is dependent upon the concentration of oxygen in the measurement gas.
As proof of small concentrations of combustible gases, such as, for example, propane gas, sewer gas, hydrogen, or carbon monoxide in an air stream, a specially designed primary element was developed with an oxygen-ion-conducting dry electrolyte and two electrodes mounted on it, made up of metal layers, in which one electrode supports an overlay of an oxidizing catalyst. Such an apparatus is described in DE-PS No. 29 18 932. The oxidizing catalyst which is not in direct contact with the metal electrode, is described as containing at least one component of the group consisting of oxides of V, Cr, Mo, W, Fe, Ni, Co and Mn and the elements Pt, Ru, Rh and Pd. The electrodes can be covered by an electrically insulating layer of MgO and an activated carbon layer on top of that. Detection of the combustible gases occurs in such a manner that the combustible gas in the air stream which is to be detected is adsorbed equally or more strongly than the air-oxygen at the metal layers of the electrodes, and that the combustible gas is oxidized, with the oxygen contained in the air stream, at a temperature between about 250.degree. C. and 450.degree. C. on the spatially divided catalyst. In this process, therefore, all combustible gases present in the air stream are fully oxidized, the oxygen contained in the air stream reduced in its concentration, and the reference gas, which is required for reception of an electrical signal, created only after the oxidation reaction. The process that is deducible from DE-PS No. 29 18 932 thus allows only the detection of individual combustible gases in an air stream or, in the simultaneous presence of several combustible gases in the air stream, the determination of the sum of the concentrations of these combustible gases. Concentrations of noncombustible gaseous components in gas mixtures cannot be detected with the process and the measuring sensor from the above German patent publication. Nor is it possible to simultaneously and selectively detect different gaseous components (both combustible as well as noncombustible) with the process and the measuring sensor from the above German patent publication.